docs: def pos_along(self, pct):
          """Return coordinates for where a Pawn should be if it has travelled
          along ``pct`` of my length (between 0 and 1).
  
          Might get complex when I switch over to using beziers for
          arrows, but for now this is quite simple, using distance along
          a line segment.
  
          """
          if pct < 0 or pct > 1:
              raise ValueError("Invalid portion")
          (ox, oy) = self.origin.center
          (dx, dy) = self.destination.center
          xdist = (dx - ox) * pct
          ydist = (dy - oy) * pct
          return (ox + xdist, oy + ydist)